Military Base, MD: Blue Nite & Blue PRODenitrification added to sand filter-based phosphorus removal for dual nutrient removal.
Location: Military Base, Maryland
Project Type: Department of Defence (DOD) Sewage Treatment
Completion Date: September 2011
Average Daily Flow 0.6 MGD
Peak Daily Flow 1.7 MGD
TP: under 0.3 mg/L
NOx-N: under 1.5 mg/L
TSS: under 5 mg/L
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Project Background & Challenges
While nutrients are a natural part of the Chesapeake Bay ecosystem, excessive amounts of nutrients, namely nitrogen and phosphorous, are of major concern. They reach the bay via multiple sources, fueling the rapid growth of algae blooms. When these algae blooms die and decompose, the resulting low-oxygen conditions, known as “dead zones”, creates harmful environments for fish, shellfish and other aquatic life. According to the Chesapeake Bay Restoration Program, the Chesapeake Bay watershed has seen its dead zone reach 3.1 cubic miles in recent years, or the equivalent of approximately 5.2 million Olympic-sized swimming pools.
In late 2005, the Maryland Department of the Environment imposed new water quality standards for both nitrogen and phosphorus, effectively limiting the amount of each nutrient a WWTF could send into rivers and streams, with the expectation of full compliance within five years. This prompted engineers for one specific Maryland military base wastewater treatment facility (WWTF)–one of nearly 500 wastewater treatment facilities in the Chesapeake Bay watershed–to seek out a viable solution.
The Nexom Answer
The Chesapeake Bay Watershed Agreement, created as part of the Chesapeake Bay Restoration Project, is an initiative to remove the bay from the Section 303(d) Ocean Water Act list of impaired waters and included pollution reduction goals for several sectors, including wastewater. It committed wastewater treatment facilities located within the watershed to making the changes necessary to meet the effluent water quality standards necessary to maintain the bay’s Total Maximum Daily Load (TMDL) for both nitrogen and phosphorus.
Together with Brown and Caldwell, as well as manufacturers’ representative Sherwood Logan and Associates, Nexom proposed their Blue PRO® for phosphorus removal, and Blue Nite® for nitrogen removal, which could be implemented simultaneously using the platform of six Centra-flo® continuous-backwash upflow sand filters with associated ancillary equipment to achieve the necessary nutrient removal.
The upgraded plant needed to comply with the new enhanced nutrient removal discharge limits of 4.0 mg/L Total Nitrogen (TN) and 0.3 mg/L Total Phosphorus (TP) as required by the Chesapeake Bay Restoration initiatives.
Since the same dosing and continuous-backwash mechanics enable both Blue PRO reactive filtration and Blue Nite’s attached growth biological denitrification, the two processes can take place simultaneously in the same tanks for total nutrient removal. Only the Blue Nite’s carbon-dosing system is required as additional infrastructure. By implementing the Blue PRO and Blue Nite systems simultaneously, the Department of Defence (DOD) wastewater treatment facility was able to comply with the water quality standards for the Maryland Department of the Environment, for both nitrogen and phosphorus levels, by 2011.
How the Centra-flo sand filter works:
The existing DOD wastewater treatment plant needed to remain in continuous operation and compliant with all regulations, throughout the entire construction period. Nexom provided the DOD military base with six Centra-flo sand filters, capable of handling an average loading rate of 1.39 GPM/SF, a maximum daily loading rate of 3.94 GPM/SF and a peak hour loading rate of 5.8 GPM/SF. The design flow of the existing plant was 5.0 MGD. Each filter is approximately 8’ D x 18’ H with a footprint of approximately 300 square feet.
Centra-flo is an upflow gravity filter of the moving bed design, providing a continuous supply of filtered water without the interruptions of backwash cleaning cycles. Influent enters the center of the filter through a central feed chamber and flows through the media bed before exiting the filter. Solids captured in the filter bed are drawn downward with the sand into the suction of an airlift pump. The turbulent, upward flow in the airlift provides a scrubbing action that effectively separates the sand and solids before discharging into the filter washbox. The washbox is a baffled chamber that allows for gravity separation of the cleaned sand and the concentrated waste solids. This process is accomplished by utilizing filtered water to clean the contaminated sand. From here, the regenerated sand is returned to the top of the filter bed, and the solids, or “reject”, are piped to a suitable disposal point.
The Centra-flo sand filter is Title 22-approved, referring to California’s Code of Regulations guidelines for how treated and recycled water is discharged and used. Discharge standards for recycled water and its reuse are regulated by the 1969 Porter-Cologne Water Quality Control Act and the State Water Resources Control Board’s 2019 Water Recycling Policy.
However, Title 22 lists specific uses allowed with disinfected tertiary recycled water (such as irrigating parks), specific uses allowed with disinfected secondary recycled water (such as irrigating animal feed and other unprocessed crops), and specific uses allowed with undisinfected secondary recycled water (such industrial uses). Other allowed uses of disinfected recycled water include irrigation of food crops and residential landscaping, supply of recreational impoundments for unrestricted body contact, air conditioning, commercial laundry, decorative fountains, and toilets.
Removing Nitrogen with Blue Nite:
Blue Nite installations across North America consistently beat nitrate permit levels as low as 1 mg/L, and the military base was no exception, easily maintaining its TN target of 4.0 mg/L. In the Blue Nite system, a carbon source is dosed to wastewater influent prior to entering the filters, where the composition of bacterial population depends on the type of carbon utilized, and continuous-backwash allows heterotrophic bacteria to grow on the media’s surface, where it converts nitrates (NO3- and NO2-) to atmospheric nitrogen (N2). The carbon source is evenly distributed, with proven platform and patented controls for minimal system upset.
The Blue Nite system is available in several configurations, and its modular nature allows easy system engineering and expansion. The filters are available as freestanding fiberglass or stainless-steel tanks, or can be configured inside concrete cells. Control systems and smaller filters may also be added.
Phosphorus reduction using Blue PRO:
Similarly, a Blue PRO reactive filtration system uses sand coated with ferric oxide (HFO), which attracts and reacts with phosphorus and metals. In the Centra-flo system, sand moves downward by gravity to an airlift device. The filtered water exits near the top of the filter, while the airlift transports phosphorus-laden media up into the washbox where the HFO coating and adsorbed contaminates are separated from the media. Water velocity is designed to carry away contaminates, while also allowing media to fall to the filter bed. The freshly scrubbed media is then recoated with HFO as its cycle begins again. Thanks to its continually-regenerative process, Blue PRO media is not consumable, as the process imparts a temporary chemical coating to the media that is then stripped and recoated cyclically inside the filter.
The Blue PRO process consistently meets ultra-low effluent phosphorus levels, even with permits prescribing less than 0.02 mg/L. For this military base, the system maintains 0.3 mg/L TP or lower, with minimal chemical dosing or waste thanks to Nexom’s proprietary reactive filtration. Many older sand filter designs were prone to media loss. However, washbox design, unique controls and monitoring systems, mitigates this concern. Both the Blue Nite and Blue PRO systems will waste almost no media in their lifetime, making top ups unlikely, and media should never need to be replaced.
Upgraded System Performance
Construction began on the upgrades in January 2010 and was completed in September 2011. Commissioning and start-up of the new components was also completed in September, along with operator training. Since adding both Blue PRO and Blue Nite to their wastewater treatment plant, Ithe military base has been able to consistently maintain their nutrient removal discharge limits of 4.0 mg/L Total Nitrogen (TN) and 0.3 mg/L Total Phosphorus (TP) as required by the Chesapeake Bay Restoration Program initiatives.
These commitments and efforts to minimize environmental impact from several sources, including wastewater treatment plants, have already had a significant impact. The Chesapeake Bay Program’s Watershed Model, also known as the Chesapeake Assessment Scenario Tool (CAST), provides nutrient pollution load estimates for the Program. According to their website “as of 2018, nutrient pollution-reducing practices are in place to achieve 39% of the nitrogen reductions and 77% of the phosphorus reductions necessary to attain applicable water quality standards as compared to the 2009 baseline established by the U.S. Environmental Protection Agency (EPA) as part of the Bay TMDL”. According to CAST, this drop in estimated pollution loads is mostly due to environmentall conscious upgrades to waste treatment facilities like this one.
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